1. FIELD OF THE INVENTION
The present invention relates to an internal combustion engine which is intended to reduce the amount of harmful exhaust gases which give rise to public pollution.
2. DESCRIPTION OF THE PRIOR ART
It has been known that the amount of the harmful gaseous combustion products, such as nitrogen oxides (NO.sub.x), carbon monoxide (CO) and hydrocarbon (HC), exhausted by an internal combustion engine is closely related to the air-fuel ratio of air-fuel mixture supplied to the engine for combustion therein. It is also known that the combustion of air-fuel mixture at a large air-fuel ratio (i.e., lean mixture) reduces the amount of the harmful exhaust gases compared to an air-fuel mixture at a small air-fuel ratio (i.e., rich mixture). However, since the ignitability of the air-fuel mixture by a spark plug is very limited by the air-fuel ratio, it is impossible to produce ignition by the spark plug when the lean mixture is at an air-fuel ratio outside the limited range. Thus, in order to reduce harmful exhaust gases by means of the lean mixture combustion, some other means for igniting the lean mixture is required. It has been known that a stratified combustion system is one of the means for enabling the ignition of the lean mixture. The stratified combustion system has such an arrangement that a stratified rich mixture is formed and held adjacent the electrodes of a spark plug in the comnbustion chamber while a stratified lean mixture is formed and held in the other parts of the chamber, which is ignited by a torch jet produced by spark-ignition of the rich mixture.
In general, the stratified combustion can be effectively performed in a so-called "torch ignition engine" which comprises a main chamber for receiving the lean mixture, a divided or auxiliary combustion chamber for receiving the rich mixture and a spark plug having electrodes disposed in the auxiliary combustion chamber. The achievement of stratified combustion is due to the fact that the division of a combustion chamber into two chambers advantageously assures the stratification of the two kinds of mixtures. However, it will be apparent that, even in a torch ignition engine of such a structure, the achievement of stratified combustion depends generally upon the extent to which the two stratified mixtures are maintained. Namely, in order that the stratified combustion may accomplish its best performance, it must be assured that an ignitable rich mixture be held within the divided combustion chamber while a lean mixture at an intended air-fuel ratio is held within the main chamber.
A typical torch ignition engine of the prior art is provided with a divided combustion chamber having a suction aperture for the flow of rich mixture thereinto and an additional intake valve solely for use with the divided chamber. The engine is so designed that the additional intake valve is opened during the suction stroke to allow the rich mixture to be directly introduced into the divided combustion chamber. The additional intake valve, however, inevitably complicates the cylinder head of the engine with a corresponding increase in the cost of manufacture of the engine.
U.S. Pat. No. 3,382,850 shows a torch ignition engine which does not require any additional valve for use only with a divided chamber. The engine, however, is not of the type that a rich mixture is directly introduced into its divided combustion chamber during a suction stroke but is of a design that the rich mixture is introduced into the divided chamber during the compression stroke. The divided combustion chamber is simply provided with a single or a plurality of discharge apertures opened to the main chamber. The discharge aperture or apertures are disposed at a substantial distance from the rich mixture supply passage. These facts lead to the problem that residual gases cannot be scavenged out of the divided chamber sufficiently to enable the chamber to be supplied with rich mixture of an amount that assures a reliable succeeding ignition, with a result that the engine does not assure a reliable continuous stratified combustion.